registerStudent(ident uid, ident id, ident studentId) {
  assume //accounts/account[uid]/student/id = studentId;
 
  assume exists //exercises/exercise[id];
  assume exists //accounts/account/student[studentId];
 
  assume not exists //exercises/exercise[id]/students/student[studentId];
 
  insert //exercises/exercise[id]/students <student id=[studentId] />;
}